The present invention relates to an earthquake isolation floor and more particularly to an earthquake isolation floor particularly adapted for keeping machinery and tools, etc. to be mounted thereon free from vibration such as caused when they are subjected to an earthquake, or the like.
For instance, electronic computers, emergency dynamos, dangerous objects (e.g. dynamite, chemicals, etc.), etc. should continue to operate or be stored in safety even during an earthquake. From this poinrt of view there is considerable attention being paid to anti-earthquake or anti-vibration means for support frames to mount machinery and tools, etc. and for places for storing dangerous objects.
Countermeasures against vibration can be divided into two kinds at present:
(1) such equipment as atomic energy plants, spherical tanks, electrical substations, etc. are originally designed against earthquakes by examining them structurally so that they will have sufficient strength against seismic input, and increasing their strength.
(2) Machinery and tools, dangerous objects, etc. are secured on a floor which is mounted on springs such that the springs absorb the seismic input when such occurs so that the machinery and tools, etc. can be kept free from danger, such a floor being referred to as a so-called "earthquake isolation floor".
The present invention relates in particular to the latter, i.e. the "earthquake isolation floor".
Hitherto earthquake isolation floors have comprised a sliding plate, a support frame slidably mounted on the plate with low friction elements being interposed therebetween, a number of springs disposed horizontally between the support frame and a foundation on which the sliding plate is laid, a floor mounted on the support frame through springs disposed vertically, a number of dampers disposed vertically between the support frame and the floor, and a latch means to secure the vertical springs during normal periods. In this earthquake isolation floor the vertically arranged springs are constituted such that they are brought into operation only after the latch means is released due to the onset of a seismic input greater than a predetermined value.
Thus, in this hitherto known earthquake isolation floor, upon the onset of a seismic input greater than a predetermined value, the latch means is released to cause the vertically arranged springs to be operated, but, the reset of the vertically arranged springs as necessitated after the earthquake has ceased is very difficult, and, further, the reset is rendered impossible should the vertically arranged springs have become misaligned with their spring seats following a large seismic input, and the floor will be placed in an unstable state. Moreover, it is complicated in constitution, and having no dampers in the horizontal direction, the damping of vibration caused by the occurance of an earthquake is delayed. Further, since the center of gravity of the earthquake isolation floor after securing the machinery and tools, etc. thereon becomes high, the floor may be subjected to a rocking vibration during an earthquake. In addition, since, with horizontally directed earth tremors, not only the horizontally arranged springs, but also the transversal rigidity of the vertically arranged springs has a large influence on the vibration prevention properties of the floor, the establishment of the spring constant in the horizontal direction is difficult and it is difficult to estimate the effectiveness of the floor during an earthquake.